1. Field of the Invention
This invention relates to a digital control system for a vibrating structure gyroscope of the kind having a vibrating structure, primary and secondary drive means for putting and maintaining the vibrating structure in vibratory resonance and primary and secondary pick-off means for detecting vibration of the vibrating structure.
2. Discussion of Prior Art
Known vibrating structure gyroscopes have been constructed using a variety of different mechanical vibratory structures. These include beams, tuning forks, cylinders, hemispherical shells and rings. A common feature in all these known systems is that they are required to maintain a resonance carrier mode oscillation at a natural frequency determined by the mechanical vibratory structure. This provides the linear momentum which produces Coriolis force when the gyro is rotated around the appropriate axis. Various systems conventionally are used to measure the Coriolis force depending on the actual structure of the gyroscope.
A typical conventional analogue closed loop control system for a shell like vibrating structure 3 is shown in FIG. 1 of the accompanying drawings. This system consists of two ideally independent loops, namely a primary or excitation loop 1 between a primary pick-off means 2 which acts as a motion detector output from the vibrating structure 3, and a primary drive means 4 which acts as a forcing input creating vibration in the structure 3. A secondary or damping loop 5 is provided between secondary pick off means 6 and a secondary drive means 7. The primary loop 1 is required to excite the vibrating structure 3 at its natural resonant frequency which is defined as a 90 degree phase between the primary pick off means 2 and the primary drive means 4, and to control the amplitude of the resultant signal at the primary pick off means 2 which in effect is the amplitude of the resultant vibration. Typically the phase detector 8 is used to determine the 90 degree phase relationship and an amplitude detector 9 with a reference level 10 is used to set the required primary pick off means amplitude. The secondary loop 5 is shown in a typical force feedback configuration to provide damping for the high Q rate response in order to achieve the required system performance.
The closed loop system of FIG. 1 is conventionally an analogue system and relies for much of its performance on the ability accurately to track the resonant frequency of the high Q mechanical vibrating structure and to discriminate, by the relative phasing, between wanted and unwanted or error signals.
Practical sensors operate at frequencies in the range 5 kHz to 20 kHz with Q factors in the range 2000 to 20000. This puts severe constraints on the phase accuracies of the electronic control systems used to implement these gyroscopes. In these systems a phase error of 0.5 degrees can lead to large bias errors and consequently failure to meet the required specification.
Traditionally the control loops for these sensors are implemented using precision analogue electronic circuits which are notoriously difficult to specify, design and integrate into small low cost systems (i.e. ASICS). It is also difficult to apply calibrations and compensations to systems based on analogue circuits. In addition, modern systems require the sensor outputs to be available in digital format to simplify system integration and enable further compensations to be applied to enhance performance.
In the FIG. 1 arrangement the primary loop 1 also includes a filter 11, a voltage controlled oscillator (VCO) 12, a gain control 13 and an amplifier 14. The secondary loop 5 includes an amplifier 18, a filter 15, and a demodulator 16 from which issues a direct current output signal 17 proportional to the applied angular rate.
There is thus a need for a digital electronic based implementation of the loops 1 and 5 in order to facilitate ASIC development, system integration and provide a route to higher performance by enabling more complex and "in loop" compensations to be applied. Unfortunately on a signal at 20 kHz a 0.5 degree phase resolution is equivalent to a sampling delay of 70 nanoseconds. This puts very high demands on any conventional precision digitisation and processing system required to resolve to the desired accuracy.
A conventional sampled data system is shown in FIG. 2 which utilises digital processing. The conventional system of FIG. 2 utilises analogue to digital converters 19 for sampling and converting output signals respectively from the primary pick off means 2 and secondary pick off means 6. Additionally provided are digital to analogue converters 20 with the converters 19 and 20 being synchronised to a fixed frequency crystal oscillator 21. The oscillator 21 operates a very high frequency (14 MHz) and the vibrating structure 3 of the gyroscope operates at a much lower frequency (20 kHz). The outputs from the primary pick of f means 2 and secondary means 6 are digitised by the digital converter 19 at a very high frequency and are fed to input 22a of a digital processing unit 22 to produce a quantised representation of the analogue output waveform. This typically requires a 70 nanosecond, (14 MHz) sample/conversion rate for each channel simply to quantise to a 0.5 degree resolution. Significant additional processing would be required to resolve the phasing sufficiently to achieve performance. This would be difficult and costly to implement to the require amplitude resolution which is typically 12 bits simply to resolve 1 degree/second. In the conventional system of FIG. 2 a clock signal is provided from the oscillator 21 to the unit 22 at 22b, a data output signal at 22c and a digital rate output signal at 22d.
There is thus a need for a digital control system which does not require very high sample rates and which is therefore more suitable as a vibrating structure gyro control system.